The present invention relates to an improved lifter structure and method of using that structure in the ejection of molded articles from an injection molding machine and the like.
It has become increasingly popular to manufacture automobile and truck trim and body pans such as fenders, bumpers. rocker panels, body panels, doors, filler panels, wheel covers, dash boards, arm rests, chin spoiler and other parts via injection molding techniques. Moreover, to minimize the emission of VOC compounds and to improve upon the aesthetic appearance of many parts, plastic parts having a paint film laminate covering the show surface have been successfully utilized in auto and truck part manufacture.
Film covered plastic parts are normally formed via a process in which the paint film laminate is insert molded with the desired thermoplastic in such manner to fuse the film over the injection molded part. The resulting injection molded part is ready for assembly without subsequent painting.
The paint film laminate used in these insert molding techniques may comprise a backing sheet of about 0.020xe2x80x3 in thickness to which paint layers are adhered to form a colored side of the laminate. Typically, the backing sheet comprises an extruded thermoplastic sheet.
In such paint film laminates, a paint film, comprising a cast dried continuous paint coating is provided over the backing sheet. The paint film may consist of a monocoat, a clear coat over a base coat or a clear coat and a base coat with interposed print or design. The paint film, including base coat, clear coat and print or design, if desired, commonly ranges from about 0.5-4 mil. in thickness.
The laminated paint films are typically provided in a roll, unwound, then trimmed to a proper xe2x80x9cpreformxe2x80x9d size and shape, ready for insertion into the injection mold. The preform is usually placed along the cavity side of the mold with the painted side thereof facing the mold cavity surface. In some instances, the preform may be placed along the core side of the mold. The mold is then clamped and the desired molten resin is injected into the mold cavity. Heat and pressure conditions in the mold partially melt the backing sheet and a melt bonding or fusion of the injected resin and film occur. Injection molds used for these processes are rear or edge gated so that the molten resin is directed along the backside of the film.
After the part has been cooled in the mold, and one of the mold cavity support surfaces is displaced from the other, the part is then ejected by the use of pneumatically or hydraulically actuated lifters and the like. The lifter cooperates with one side of the mold (usually that side contiguous with the plastic substrate) and ejects or xe2x80x9cpopsxe2x80x9d the finished part out of the mold so that the molding production cycle can continue with the insertion of paint film laminate and plastic into the mold cavity. In many instances, the aesthetic appearance of the paint film laminate covered side of the part, normally used as the xe2x80x9cshowxe2x80x9d side of the part, exhibits a surface blemish in the region in which the flowing plastic first impinges upon the lifter surface.
When elongated paint film covered parts are produced, such as in the case of rocker panels, chin spoilers, bumpers, etc., the blemish can take the form of an unsightly surface line or bump on the show side of paint film laminate covered side on the part, extending transversely to the longitudinal axis of the elongated part, causing a difference in gloss or light reflectance between neighboring sides of the paint film surface adjacent the bump.
The present invention is accordingly directed to methods and lifter apparatus for ejecting plastic parts and specifically paint film laminate covered plastic parts from a mold cavity that inhibit the formation of surface blemishes during the ejection step of the part in the molding sequence.
In conventional injection molding apparatuses and methods, the lifter comprises a plate like structure having a leading edge that is perpendicularly oriented relative to the longitudinal axis of the elongated part. After the part has been formed in the mold, one half of the mold is normally retracted and the lifter plate associated with the other mold section is actuated via hydraulic or pneumatic pistons to strike the part and eject it from the mold cavity.
The present invention pertains to methods of molding an automobile or truck plastic part wherein a mold cavity is formed by at least two mold portions. A charge of plastic is injected into the mold cavity along a upstream to downstream direction by a runner that is provided in one of the mold portions. After the part is molded in the mold cavity, one of the mold cavity portions is separated from the other to thereby open the mold cavity . The part is ejected from the mold cavity by contacting it with the lifter member having an angled edge portion located along an upstream boundary of the lifter and a trailing edge portion located downstream from the leading edge.
The method is particularly advantageous in the formation of plastic parts of the type wherein a paint film laminate is superposed over at least a portion of the substrate plastic and forms a film laminate covered show face of the part leaving a rear face comprising the exposed plastic surface. In such cases, the lifter strikes the rear face of the formed part.
It has been found that perceptible blemish formation on such paint film laminate covered plastic parts is minimized by providing a leading edge of the lifter that is disposed at an oblique angle relative to the longitudinal axis of the part. Preferably, this oblique angle is from about 30 to 60 xc2x0 and in the presently preferred embodiment, is about 45xc2x0.
In accordance with conventional co-molding of paint film laminate covered parts, the desired paint film laminate is provided along one surface of the mold cavity with the injected plastic entering the rear of the cavity and fusing with the paint film laminate. The parts formed are of generally elongated nature and have a longitudinal axis extending therethrough. During the molding operation, the charge of plastic material is injected into the mold cavity usually along the longitudinal axis of the part (and the mold) from an upstream to a downstream direction along the back of the paint film laminate.
After the mold portions have been separated. the rear side of the part (opposite from the paint film laminate show side) is struck by the lifter plate in accordance with the invention. The provision of the obliquely angled upstream or leading edge of the lifter has been shown to minimize perceptible blemish formation in the finished part. Although applicant is not to be bound to any particular theory of operation, it is thought that the shear forces that act along the flowing plastic material are substantially decreased by provision of this angled leading edge of the lifter compared to the prior art. In contradistinction. in prior art devices where the leading edge of the injector is oriented perpendicularly with regard to the longitudinal axis of the part, excessive shear of the plastic leads to a turbulent flow in the region proximate the leading edge of the lifter with the plastic thereby forming a perceptible bump in the paint film show face of the part. Either side of the bump reflects light differently, causing an undesirable gloss difference along the length of the part resulting in characterization of the part as a reject.
The apparatus in accordance with the invention thereby comprises a first mold part and a second mold part which, when in contiguous relation one to another, define a mold cavity there between. One of the mold parts is separated from the other via pistons or other linear movement actuators such as a ball screw or equivalent to open the mold cavity. The lifter in accordance with the invention is associated with one of the mold parts and is adapted to contact the plastic part after the first and second mold parts have been separated to eject the part from the mold cavity.